overwintering rosemary, indoors and out

THIS ONE COMES UP EVERY FALL (especially from gardeners in Zones less favorable for it than 7ish): How do I keep my rosemary plant happy all winter? The answer: Cool sunshine—that rarest of indoor conditions in a heated house in the year’s darkest days, and not a guaranteed steady diet weather-wise outdoors many places, either. How to accommodate Rosmarinus officinalis, outdoors or in.  

Start with a cold-hardy cultivar if you plant to try to overwinter rosemary in the ground in other than a truly frost-free hardiness zone. ‘Arp’ is the best known, along with ‘Hill Hardy’ (also known as ‘Madalene Hill’ after the late herb gardener from Texas; ‘Arp’ was her discovery, by the way, the result of her search for plants that could take not extremes of cold but the Texas heat). Oregon-based Nichols Garden Nursery’s owner touts ‘Nichols Select’ as being a toughie, too.

It’s “as hardy as any I’ve grown, probably Zone 6B, and the flavor is terrific,” Rose Marie Nichols McGee in an interview one spring. “It was planted 25 years ago at our home and survived minus-7 degrees F once. I think this is your best for a long-lived rosemary.”

The U.S. National Arboretum website reports on trials of more than four-dozen cultivars of rosemary, and how they fare on all scores. Even in USDA Zone 7A, the gardeners there take the precautionary measure of taking cuttings from the collection, in case winter causes havoc that cannot be anticipated with the herb-garden inhabitants. (I pinch or prune my rosemary back a bit–not to start cuttings, but to freeze the delicious sprigs for use in winter like this, and keep the plant tighter.)

Proper location and care will help keep the well-chosen plant happy. Nichols McGee advises “helping it along with some lime or bone meal worked into the soil and provide good drainage,” and planting rosemary in a well-drained position with six hours a day or more of direct or very slightly filtered sunlight. Many Zone 6ish Northerners optimistically try one of those extra-hardy cultivars planted in a sheltered spot, such as by a wall, and then mulch heavily after frost.

“What is hardest on our woody plants, including rosemary,” she adds, “is wet soil followed by temps that drop way down near zero then warm and drop again with considerable frost heaving.” In her Pacific Northwest location, she says they don’t mulch around the trunk and root zone, “but do make generous use of garden blankets, throwing one or two layers over plants we’re worried about.”

For container-grown plants, she says, whenever temps are going down to the low 20s all rosemary will benefit from protection. “If you have a garage or cool porch, pull your plant into this sheltered area until the harsh cold passes (which might be all winter in the colder zones; make sure the garage or porch will stay a safe temp throughout the needed timespan). Move it back to its regular position when temperatures moderate.”

So what if there is no such spot, if even with a temporary garden blanket it’s just too tough outside where you live? Here’s where the “cool sunshine” trick really gets tricky. You’ll be looking for a spot near a sunny, southern window (west might also work), where the temperature is around 50 to 60 degrees.  My mudroom is right, I think, so that’s what I will try—but many of us don’t have enough light in winter. Think about adding a supplemental plant light–which can really help with many tender things that keep their leaves year-round like rosemary. We’ve all seen rosemary and other evergreens being tortured indoors with too little light get all stretched out, or etiolated.

Have a rosemary in the ground that you fear should have been in a pot, so it could come indoors? A third option would have been to dig it and pot it up for the winter, but that’s best done around Labor Day, to give the plant a chance to adapt to its new tight quarters before the added shock of indoor life begins. A very fast-draining mixture–perhaps with a few inches of gravel at the bottom of a generous-sized pot for added insurance–is best.

Very important: Don’t overwater. Remember, no wet feet, especially in winter—whether that’s inside or out, in a pot or in the ground. And again: Freeze some sprigs, just in case.